It is known that anchor rods are usually used for the support of walls, bulkheads and the like, being limitedly sunk into the ground or resting on loose ground, for example for the forming of scarps, especially in road and railway works.
Such rods are generally anchored, with their inner end, to the surrounding ground, at the bottom of a hole made into the ground for housing the rod itself, said rod end being buried in a casting of cement mixture, forming therewith an anchoring bulb. The same rods comprise, at their outer end, a head-piece for connection to the wall or like to be supported, and in correspondence of their intermediate portion, a number of free steel strands under tension, contained in a protection pipe. According to known technique, the inner end of such rods is formed by simply deforming with undulations the end parts of the strands, extending beyond the protection pipe as far as the bottom of the hole for the rod, and by fastening crosswise such deformed parts of the strands, so as to form a branched structure, adapted to efficiently engage with the casting of cement mixture, which is meant to embody it so as to form therewith the anchoring bulb of the rod.
In the setting up, after placing the rod inside the hole provided therefor, one forms the anchoring bulb by injecting cement mixture under pressure to the bottom of said hole, hence obtaining the anchoring of the rod to the ground, preferably a rocky soil. Subsequently, the free length of the strands is put under tension, by firmly connecting said strands to the outer head-piece of the rod, which is in turn fixed to the wall or like to be supported.
This system, generally adopted up to now, has drawbacks which are not negligible. In fact, when putting under tension the steel strands of the rod, in correspondence of their deep part which is buried in the hardened cement mixture with which they form the anchor bulb, such strands cause in such mixture, by adhesion, an elongation which is equal to the elongation of the actual strands. Said elongation is remarkable in the less deep part of the bulb, because of the high unitary stress in the strands, and this is hardly compatible with the characteristics of cement mixture. This easily causes the forming of more or less capillary crack in the anchor bulb, which crack are apt to let through any aggressive liquids which might eventually be present in the surrounding soil or rock. Even the bulb-rock adhesion force, which is initially exerted in the less deep part of the anchorage, often reaches its peak value, causing even in the rock, crack which reduce its resistance and facilitate the seepage of aggressive agents.
The cracks cause the depthwise extension of the adhesion stresses between the steel and the hardened cement mixture, and between the latter and the rock, and the whole bulb may hence fall apart, if it is of reduced length.
To eliminate the above drawback, anchor rods have been set up making use of steel bars, in replacement of the strands, said bars being placed in the area of the bulb, within a cylindrical metal body, and being screwed to the end of said body which is arranged at the bottom of the hole for the rod.
Also in this case, the anchor bulb is obtained by injecting cement mixture under pressure into the bottom of the hole. In this way, the pull of the bars is applied on the very end of the rod, widening the extension of the surface of actual adhesion to the rock, in correspondence of which there could be creeping of the bulb and settling of the surrounding ground, and creating a more favorable distribution of the stresses in the cement mixture of the bulb and in the contacting rock, which are compressed and not pulled. In this way, the above drawbacks are reduced, though not yet to a satisfactory extent.